The present invention relates to an activated carbon, a process for the preparation thereof, and an electric double layer-type capacitor electrode. More particularly, the present invention relates to an activated carbon having specific properties and utility, specifically the activated carbon suited for purification of service water, treatment of waste water, refining of foods, and use as an electrode material for capacitors, a process for producing such activated carbon, and an electrode for electric double layer-type capacitors.
Request is rising in the industries for activated carbon having higher utility and functional potentiality, but available activated carbons for the said uses are still unsatisfactory in their properties, and can not meet the request.
Attentions have been focused on electric double layer-type capacitor for use as back-up or auxiliary power source. Particularly, an electric double layer-type capacitor using activated carbon or porous carbon as polarizing electrode has an excellent performance and the demand of the electric double layer-type capacitor is said rising sharply with the progress in the field of electronics. Recently, efforts are being made not only for the utilization of the said capacitor for miniaturized parts as conventional memory back-up power source but also for the development of the large-capacity products such as those used for auxiliary power source for motors.
The principle of electric double layer-type capacitor has been known for long, but it is only recently that this electric double layer-type capacitor has been put to practical use.
It has been known that the electrostatic capacitance of electric double layer-type capacitor is subject to change depending principally on the surface area of the polarizing electrode having an electric double-layer formed thereon, the electric double-layer capacity per unit area and the electrode resistance, but its relation with the size of the ions in the electrolyte for forming the electric double layer is of particular importance.
Specifically, in an organic solvent-type capacitor utilizing ammonium ions, phosphonium ions or the like, it is said that the specific surface area portion with a pore diameter of not less than 20 .ANG. in the pore distribution determined from the nitrogen adsorption isotherm is associated with electrostatic capacitance of the said capacitor. It is also considered that in the aqueous solution-type capacitor using an aqueous sulfuric acid solution as solvent, the specific surface area portion of the pores with a pore diameter of not less than 20 .ANG. contributes to electrostatic capacitance under a condition of high current density and low temperature.
Hitherto, increase of specific surface area of activated carbon has been accomplished by an activation treatment with water-vapor or chemicals, but in activated carbon used for electric double layer-type capacitors, the utilization is made of the specific surface area of the pores with a pore diameter of not less than a specified value, so that activated carbon having an efficient pore distribution has been desired.
Thus, the key to the improvement in performance of activated carbon used for electric double layer-type capacitors is to efficiently produce an activated carbon with many pores of specific surface area with a pore diameter of not less than 20 .ANG. in the pore distribution determined from the nitrogen adsorption isotherm.
Increase of the double layer capacitance has been attained by enlarging the specific surface area of activated carbon. Generally, however, an activated carbon with a large specific surface area has only a few mesopores with a pore diameter of not less than 20 .ANG., although it is rich with micropores. The mesopores can be increased by furthering activation, but this furthering of activation tends to decrease the whole specific surface area. Accordingly, there has so far been available no activated carbon which has realized the efficient utilization of specific surface area, in which the specific surface area of the mesopore region required for a large-capacitance capacitor is sufficiently large while the micropore region which little contributes to capacitor performance is reduced to a minimum. Generally, activated carbon with a large specific surface area can be obtained by activating a carbonaceous material such as coal, coconut shell, sawdust, etc., by a chemical means, and the obtained activated carbon has a specific surface area of the whole pores of not less than 3000 m.sup.2 /g. Among these activated carbons, there are activated carbons in which the specific surface area of the pores with a pore diameter of not less than 20 .ANG. is about 1400 m.sup.2 /g, but since there also exist the micropore portions which little contribute to capacitor performance in use as activated carbon for capacitor, their performance in terms of capacitance was not satisfactory.
Although activated carbon with a large specific surface area can be obtained by a conventional activation method such as water-vapor activation or chemical activation of a carbonaceous material such as coal, the obtainable specific surface area of pores with a pore diameter of not less than 20 .ANG. which is called mesopore region in the pore distribution determined from the nitrogen adsorption isotherm, is up to about 1000 m.sup.2 /g even if the activation degree is controlled. Further, the ratio of the specific surface area the pores with a pore diameter of not less than 20 .ANG. to the specific surface area with the whole pore diameter was very low.
The pore distribution inclines to the micropore side by repeating the activation treatment, but repeated activation poses an economical problem. The specific surface area of the pores with a pore diameter of not less than 20 .ANG. can be increased temporarily, but the ratio of the specific surface area of the pores with a pore diameter of not less than 20 .ANG. to the specific surface area of the whole pore diameter is still low and satisfactory activated carbon can not be obtained.
In view of the above, there is yet found no activated carbon which can be effectively applied to large-capacitance capacitors, and it is considered necessary to design and prepare the pore distribution with a new concept.
As described above, in making activated carbon suited for large-capacitance capacitors, it has been impossible with the conventional concepts to increase the specific surface area by furthering activation to an excessive degree and to promote formation of useful mesopores.
As a result of the present inventors' earnest studies on a method for controlling the pore distribution, for obtaining an activated carbon which has made best use of surface area, in which the specific surface area of the mesopore region with a pore diameter of not less than 20 .ANG. in the pore distribution determined from the nitrogen adsorption isotherm necessary, for large-capacitance capacitors is sufficiently large while the micropore region with low potency of contribution to capacitor performance is minimized, it has been found that by (1) carrying out a water-vapor activation of a carbonaceous material and subjecting the thus obtained carbonaceous product wherein the ratio of the specific surface area of pores with a pore diameter of not less than 20 .ANG. to the specific surface area of whole pores is not less than 0.30, to an alkali activation-treatment, or (2) carbonizing a carbonaceous material, subjecting the carbonized carbonaceous material to an oxidation treatment and then further subjecting the oxidized product to alkali activation, the thus obtained activated carbon has the desired pore distribution which has never been obtainable with the prior arts, wherein the specific surface area with a pore diameter of not less than 20 .ANG. in the pore distribution determined from the nitrogen adsorption isotherm is not less than 1000 m.sup.2 /g and the ratio of the specific surface area of pores with a pore diameter of not less than 20 .ANG. in the pore distribution determined from the nitrogen adsorption isotherm to the specific surface area of whole pores is not less than 0.45. On the basis of this finding, the present invention has been attained.